Zone Air Temperature is exceeding the value within schedule

sezzeldin · Posts: 14

Hi,
I'm trying to simulate a single zone office space in a very hot country (Bahrain) with outdoor temperature that could reach 45C during summer.
I'm using compact HVAC system (VAV with terminal reheat) with adequate sizing in order to obtain the design zone temperatures without any system constrains.
I set the scedule to maintain 24C as a cooling setpoint during the working hours.
...BUT...
During summer, the zone air temperature exceeds 24C and reaches about 30C that inturn increases the Operative temperature
How can I maintain zone air temperature within the designed setpoints?
Thanks for your support,
Sherif

koen25071979 · Posts: 28

Hi Sherif,

Its propably one of the settings. Try to set the temperature control to "Air Temperature".
You can do this in the calculation options data.
Click to the options tab and set temperature control to "Air temperature".

The reason that air temperature is above the operative temperature is that temperature control is set as "operational temperature".

I had similar problems. This solved it.

Regards

Koen

sezzeldin · Posts: 14

Hi Koen,
The "Air temperature" is already selected
In case of Bahrain city, as the outdoor temeprature is very high, the zone radiant temperature is always higher than the zone air temperature which makes the zone operative in between and always higher than the zone air temperature
I've tried to change the temperature control to "Operative temperature" but always exceeding
Don't know how to maintain zone air temperature within the designed setpoints 24C?
Kind regards,
Sherif

koen25071979 · Posts: 28

Hi Sherif,

This is strange, i tried to simulate your problem in a simple building using same setpoints. With Bahrain weather data. But my simulation runs perfectly to the required setpoint.
Are you working with the latest version of designbuilder (1.5.0.068)?
If so and if youre sure that all your setpoints and shedules are correct i would suggest to upload your model so that designbuilder support team can take a look into it.

Regards

Koen

koen25071979 · Posts: 28

Hi Sherif,

Did some more testing on my simple building and found something that could be the reason.
Check your shedules of HVAC. It could be that they are not set correctly.
If the air temperature should be maintained, makesure that your VAV system is working. Check the "help" in designbuilder how to set HVAC shedules for heating, cooling and ventilation.

This could be the reason why your zone air temperature is not maintained.

Regards

Koen

sezzeldin · Posts: 14

Hi Koen,

Thank you for your concern and help

I think there is a problem related to System capacity because, with same schedules, I changed HVAC Sizing method in the Model Options from Adequate into manual sizing and specified Cooling & Heating Capacities to 1000 KW to exclude capacity problems, the temperatures didn't exceed and followed the scheduled setpoints. But When returning back to adequate HVAC sizing, the system couldn't support the high outdoor temperatures and couldn't maintain the zone air temperature within the design boundaries.

What do you think?

Sherif

koen25071979 · Posts: 28

Hi Sherif,

I don't know what could be the reason for this.
I would suggest uploading your dsb-file so that i can take a look into it. Because i can't see now which kind of settings you applied to the HVAC.

Regards

Koen

sezzeldin · Posts: 14

Hi Koen,

Thanks again for your help
I'm attaching the DSB-file
Please try to simulate the whole year (once using adequate and another using manual with 1000KW as capacity for Heating and Cooling) and focus at the zone air temperature especially on Tuesday 20th August
the temperature reaches about 30C while it was set to 24C.
Could you please justify?
Kind regards,
Sherif

koen25071979 · Posts: 28

Hi Sherif,

Yes, your building is not keeping the setpoint temperature, and propably for following reason:

The difference between air temperature, operative temperature and radiant temperature is to large.
I don't know how exactly, but i think there are some limits.

I found something on the help of designbuilder,
search in help for energyplus errors and warnings.
The third error is saying something about cooling design calculations, air temperature and operative temperature. Take a look into it.

Designbuilderteam:

Could you help Sherif on this one?
Its verry strange that the cooling can't keep the setpoint.

Regards

Koen

Andy Tindale · Posts: 1111 Location: Stroud

The problem with this model is that the cooling coil is not being adequately sized in the autosizing-simulation carried out before the main simulation to account for the very high humidty in the outside air during August in the weather data used.

This is at least partly because the ASHRAE cooling design conditions set at site level have lower outside air humidity than the values in the hourly weather file. Changing the maximum dew point temperature to 35degC results in adequately sized cooling coils as does changing the 'Compact HVAC autosizing sizing factor' on the Options tab of the simulation options dialog from 1.2 to 2.0.

Andy

sezzeldin · Posts: 14

Hi Andy,

Could you please tell me where to change this maximum dew-point temperature to 35degC?

Thanks for your help and for Koen too

Sherif

Andy Tindale · Posts: 1111 Location: Stroud

You can change the web-bulb temperature at the time of the max dry-bulb temperature on the Location tab at Site level.

Andy

sezzeldin · Posts: 14

Thank you... Andy and Koen
It's now working as planned Very Happy

Kind regards,
Sherif

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